Ideally to maximize oxygenation, the fish tank should have a large exposed surface area, but I find that in the commercial ones the roof is closed. Also is it OK if we keep the roof exposed ? I tried it but it smells awful near the surface of the fish tank.

9 Answers
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Nearly any cover you buy or build will allow enough air to circulate. If you're concerned, you can just leave a bit more space between the water surface and the cover, but this really isn't a big deal in practice. And there are some good reasons to keep a tank covered:

A cover reduces evaporation, which means you don't lose water as quickly. This can be pretty noticeable in warm water systems. Evaporation also leaves mineral deposits behind. In saltwater tanks this quickly leads to 'salt creep', where a thick salt crust forms over everything nearby; even freshwater tanks will start to look a bit gross if you don't keep that clean.

Many fish can and will jump out of the tank. Nearly any fish the swims near the surface could do this when they're startled. Some, like hatchetfish, actually prey on low-flying insects and are notoriously good at finding small gaps in a hood to jump through: in their case a hood or cover is mandatory.

Depending on your tank's light, the cover might be necessary to actually support it. If the light doesn't rest on the rim of the tank, you need something to hold it up.

On a similar note, a cover will keep foreign objects from falling into the tank. If there are young children or pets in your household, this could be a very important consideration. Same thing if you keep any supplies above the tank.

But a cover isn't mandatory in every tank. There are some very attractive open-tank setups, though the ones I've seen tend to have few top-swimming fish in them. It's just a question of knowing the tradeoffs and deciding if it's worth it to you.

My 75 gallon & 30 gallon tropical fish tanks have hood lights but are partially covered with a grated screen because the tops don't fit the entire surface area. If I didn't have them completely covered, the fish could possibly jump out.On the other hand, I have 40 gallon breeder tank with 4 aquatic frogs, 6 fancy goldfish & 6 bumblebee catfish. The tank is about 3/4 filled with water (not to the top) and I've had the tank for many years and never had a cover on it and never had a problem. But if the water was filled to the top, the frogs could possibly jump out.

so you are saying the lid on fish tanks are to avoid the fish escaping,i think this is only a part of the answer as it have other funktions as well like keeping dust out and limiting evaporation.
– trond hansenMar 25 '18 at 7:53

It is absolutely ok to keep aquarium completely open and uncovered, if you do not have a specific reason to cover it. There are many good reasons to cover an aquarium - like jumping fish, labyrinth fish that need warm and humid air above water, excessive evaporation and others. But those reasons do not apply to every aquarium. For example, the evaporation can be even a benefit, if you have air too dry in your place.

I keep most aquariums open. Aquariums in the very popular Takashi Amano style are also almost always open. In my opinion it looks better and it is very nice to be able to see the aquarium from the top.

Earthy may be, but earthy is not awful.
– Fish-in-TeaMar 27 '18 at 20:51

Smells are by their nature subjective.
– Graham ChiuMar 27 '18 at 21:44

Well, in my 30 years of fishkeeping I have never met a person who said a healthy aquarium smells awful - that way subjective it is. If there ever was a complaint there was always some problem with the aquarium itself - either a new, not properly cycled tank, or a tank that was overfished, neglected etc. In fact, I use smell as a quite sensitive diagnostic tool. Do flowerpots smell awful to you?
– Fish-in-TeaMar 29 '18 at 7:20

I don't like the smell of my fish tanks
– Graham ChiuMar 29 '18 at 9:30

In my years of experience, I find it better to keep the roof closed. This way the water evaporates slower, and it keeps the water clean. But nowadays I see more and more people leave it open with just LED lights hanging on top. I have never tried, but their tanks seem in good condition too. So maybe there are other ways to maintain the tank?

And yes to a lot of the answers here, a healthy tank shouldn't smell awful.

Welcome to Pets Stack Exchange! :) When you say 'it keeps the water clean', what do you mean? From foreign contaminants?
– HendersAug 22 '18 at 20:29

@Henders The glass top will stop most of the dust from falling in.
– CJ DennisAug 23 '18 at 1:05

@CJDennis - would you say that dust is harmful to fish? When I read about clean water, I'm thinking no ammonia, no nitrite, very low nitrates. I've never heard about dust being a problem for fish.
– HendersAug 23 '18 at 8:04

I have a canopy on my tank because I like to keep the cat out and the fish in. But apparently my Western blacknose dace enjoys headaches. He continually jumps and bangs his head on the canopy. Several times I have seen the dace and my central stoneroller minnow with the skin peeled on the top of their heads. The spotfin shiners seem to be a bit smarter. Below are my jumpers.

That said, a canopy will not affect the aeration of your tank unless you've sealed it tight. It's best to have a canopy to keep jumpers in the tank.

Most fish will not jump out. But the lids prevent water loss by evaporation. If you have an auto-fill tank, no problem. But nitrates and nitrites become concentrated with low water in the tank, so keep your water levels consistent.